Micromirror devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits. One commercially successful micromirror device is the digital micromirror device manufactured by Texas Instruments, which is used as the spatial light modulator in the DLP-branded image projectors.
Micromirror devices are primarily used in optical display systems. In display systems, the micromirror is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, micromirrors typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the micromirror surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast of the projected image.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast of images produced by the device.
In addition to the improvements to the structure of the micromirror itself, many improvements have been made in the pulse width modulation techniques used to create the perception of analog intensity levels from the purely digital device. The creation of the appearance of smooth analog intensities depends in large part on the ability of the micromirror system rapidly to switch the micromirror elements on and off to transmit very short pulses of light onto the image plane. Short bit pulses increase bit depth, or the number of data bits that may be displayed in a given frame period. As the contrast and bit depth of the projected image increases, very minor pulse width modulation errors become noticeable to the human eye are result in objectionable image artifacts. What is needed is a method and system of reducing the occurrence and effect of pulse width modulation errors.